Possible Sites for the Origin of Life

There are many different theories about where the origin of life occurred. These theories range from life beginning in deep sea thermal vents to bacterial life arriving from other places in the universe, among others. Some of these theories are more credible than others, yet all provide an interesting explanation for life's beginnings.

Significance of Water

Everyone knows that liquid water is essential for humans to survive. In fact, it is essential in the chemistry of all biological systems. Water (chem window: give link to water module) provides the medium in which the transport of molecules can occur in reactions. Because water is necessary for all life, scientists look for evidence of liquid water wherever they search for life, whether it is somewhere on Earth, or even somewhere else in our solar system or beyond. In fact, astronomers are currently examining the satellites of Jupiter, Europa and Ganymeade, and Titan, one of Saturn's satellites, to see if they contain liquid water and the conditions which may give rise to life as we know it.

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Before we look to see where life may have begun elsewhere in the universe, let's look at where, or how, life might have begun on the earth.

Thermal Vents

One current theory is that life originated deep beneath the surface of the ocean at deep sea hydrothermal vents. These hydrothermal vents were first discovered in 1979. Soon after, scientists made an exciting discovery. These vents release hot gaseous substances from the center of the earth at temperatures in excess of 572oF. Previously scientists were sure that life could not exist, deep beneath the surface of the ocean. After the discovery of hydrothermal vents, they found ecosystems thriving in the depths of the ocean. These ecosystems contained various types of fish, worms, crabs, bacteria and other organisms which had found a way to survive in a cold, hostile environment without energy input from sunlight. Because life had been found to exist where it previously was thought unable to, many scientists began to ask questions as to whether or not this was where life may have originated on the earth.

On the molecular level, the chances of life originating at deep sea thermal vents is not likely. It is known that organic molecules are unstable at high temperatures, and are destroyed as quickly as they are produced. It has been estimated that life could not have arisen in the ocean unless the temperature was less than 25oC, or 77oF.

Supporters of this theory claim that the organic molecules at the thermal vents are not formed in 300oC temperatures, but rather in a gradient formed between the hydrothermal vent water, and the extremely cold water, 4oC (39.2oF), which surrounds the vent at the bottom of the ocean.
The temperatures at this gradient would be suitable for organic chemistry to occur. Debates still remain, however, as to the gradient's effectiveness in producing organic compounds.

Extra Terrestrial Sources

Panspermia
In the early twentieth century, a Swedish chemist named Svente Arrhenius developed a theory called panspermia. Arrhenius' theory accounted for life's origins by simply stating that life did not originate on the Earth, but originated elsewhere in the universe. He believed that cellular life reached the Earth hiding inside a meteor which hit the Earth long ago. Newly uncovered evidence suggests that this might be possible, since an organism inside a meteor (Picture of impactor) would be safe from the high levels of radiation in space, and would be kept at a relatively low temperature. The odds of an organism surving inside a meteor for thousands of years, however, are not high. It is even less likely that organisms would be able to withstand the high energy impacts of bolides into the Earth or other planetary objects. Most scientists today do not look at this hypothesis as a very likely origin of life on the earth. However, it is considered possible, at least for now, and so is still a candidate for life's origin on earth.

Frozen Ocean
Three billion years ago, the Sun which lights our solar system was thirty percent less luminous than it is today. Mant people believe that if the Earth's atmosphere was the same then as it is today, the oceans would be frozen. But recently, Jeffrey Bada of the Scripps Institution of Oceanography has proposed that the oceans would not completely freeze. Instead, he calculates that only the top 300 meters of the ocean would freeze over.

You might think that icy cold water trapped under hundreds of meters of ice would not be beneficial to life beginning, but in fact it is advantageous in many aspects. One advantage is that the layer of ice would provide a protective shield by preventing ultra-violet light, which enters the earth's atmosphere and destroys organic compounds, from reaching the developing molecules. Another advantage is that it would provide safety from the devestating effects of impact frustration. ( Definition Box -Impact frustration is a theory which says that life may potentially have arisen many times, but was wiped out due to severe bolide impacts) The water beneath the ice would be cold, allowing for organic molecules to survive over much longer periods of time. These organic molecules could have been provided by the hydrothermal vents still prevalent on the ocean floor today. With a sufficient supply of organic molecules safe from ultra-violet radiation and bolide impact frustration, many believe that this was the environment allowing life to get a foothold on a hostile earth.

With a barrier between the atmosphere and the ocean, the debate concerning the composition of the atmosphere becomes much less significant. All of the components needed for organic syntheses such as the Strecker synthesis would be provided and kept stable, while the bottom of the ocean would provide a place for organics to gather and react. Following this reasoning, the atmospheric composition may only be important after life came out of the water, when life had already begun.